1. Field of the Invention
This invention relates to a secondary battery using lithium, as an active material, and more particularly to an improvement of an electrolyte to be used in a secondary battery.
2. Description of the Prior Art
Currently, carbon compositions, including graphite, coke and the like, are tested as materials for use as a negative electrode in a secondary battery using lithium. It is appreciated that the carbon compositions have a superior reversibility in charge and discharge cycles and substantially prevent mossy lithium from electrically depositing on a surface of the negative electrode. In particular, graphite has a high lithium intercalating/deintercalating capacity and therefore is considered to be one of the most useful carbon compositions as a negative electrode material. The use of graphite as a negative electrode material, however, presents a problem when graphite is used in propylene carbonate or butylene carbonate because graphite has many active spots in its structure as compared with coke. Propylene carbonate and butylene carbonate are known as solvents to be used in an electrolyte which may be easily decomposed. Therefore, if a graphite negative electrode is used in an electrolyte containing propylene carbonate or butylene carbonate as a solvent, the solvent is adsorbed onto the active spots of the graphite negative electrode and readily generate gas through decomposition. As a result, the decomposition of the solvent prevents lithium ion as an active material from intercalating into the graphite on charging the battery, and causes an increase in polarization as a result of a gas overvoltage. Consequently, a battery capacity is decreased.
Such a problem may be solved by using ethylene carbonate as a solvent, which is not easily decomposed as compared with propylene carbonate. In particular, a mixed solvent of ethylene carbonate and a low boiling point solvent having the boiling point of 150.degree. C. or lower has excellent characteristics as an electrolyte solvent to be used in a lithium secondary battery.
However, the above electrolyte including ethylene carbonate has a relatively high solidifying point. Therefore, its ion conductivity is not sufficient in a low temperature range. For example, the above phenomenon is observed when an electrolyte includes a mixed solvent of dimethyl carbonate and ethylene carbonate. Dimethyl carbonate is a low boiling point solvent and ethylene carbonate has a solidifying point (melting point) of about 39.degree. C.-40.degree. C. When they are mixed, the electrolyte may have a solidifying point of about -20.degree. C. As a result, batteries equipped with the aforementioned electrolyte have an extremely reduced battery capacity when discharging at a low temperature of -20.degree. C. or lower.